Overvoltage protection of dc to dc converters using ferroresonance

ABSTRACT

The frequency characteristic of a ferroresonant regulator is shown to have a critical jump frequency above which the output voltage of the regulator takes a sudden drop. A regulated voltage converter utilizes this discontinuity to provide automatic overvoltage protection. An inverter drives a ferroresonant regulator and a feedback loop controls the frequency of the inverter in response to output voltage. A short in the feedback loop drives the frequency above the critical jump frequency to provide automatic voltage limiting.

PATENTEDDBHI I922 SHEET 1 OF 2 FIG.

I ,PRIOR ART v II INVERTER FERRORESONANT- REGULATOR CONVERTER V T0 FREQlouT uT P HART INVENTORS R'. .11 KAKALEC WALK v ATTORNEY PATENTEDnmmsn.3.599.424

' SHEEI a ur.2

FIG. 3

ERRoR DETECTOR FERRORESONANT E REGULATED BACKGROUND OF THE INVENTIONThis invention relates to inverter circuits having a ferroresonanttransformer. More particularly, it relates to such ferroresonantinverter circuits, the output voltage of which is regulated by a voltageto frequency feedback loop.

As electronic circuits and devices have become more complex, the demandsupon what was once the simple power supply have kept pace. It is notunusual to require in a single system many different regulated voltages,all held within very close tolerances under widely varying loads, andall produced with high efficiency for low heat dissipation. In addition,since the power supplies themselves usually include semiconductordevices, they must ordinarily be self-protecting in case of a shortcircuited output.

One method of satisfying these conditions takes advantage of the highefficiency and self-protecting features of the well-known ferroresonantregulator. An'inverter drives the ferroresonant regulator to produce anoutput voltage that is regulated against changes in the input voltage. Afeedback loop varies the frequency of the inverter in response to outputvoltage. Since the output voltage of a ferroresonant regulator is afunction of the frequency, the feedback loop is closed, and closeregulation of output voltage with both input voltage and load results.Because of the leakage characteristic of the ferroresonant transformer,a short circuit across the output does no harm to either the transformeror the inverter. An example of such a system is disclosed in US. Pat.No. 3,590,362 which issued to Robert J. Kakalec June 29, 197 I.

When the load supplied by a power system includes intricate and delicateintegrated circuits, an additional vital requirement is added to thelist of requirements for the power system; the output voltage must notexceed a relatively low maximum even in the event of a power systemfailure. Even a single voltage spike can destroy the valuable load. Inthe type power system described, a short circuit in the feedback loopcan drive the frequency and therefore the output voltage above atolerable limit. A separate overvoltage protection circuit has thereforebeen required.

An object of this invention is to provide at no additional cost,overvoltage protection of a ferroresonant regulated power system.

Another object of this invention isto automatically reduce the outputvoltage of a ferroresonant regulated power system in the event of afailure in the feedback loop.

SUMMARY OF THE INVENTION This invention takes advantage of adiscontinuity in the frequency characteristic of a ferroresonantregulator circuit. When the frequency is raised to a critical jumpfrequency, often two or more times the usual operating frequency, theferroresonant regulator switches to a nonsaturating mode, and the outputvoltage drops considerably. In a voltage regulating system of the typein which an inverter drives a ferroresonant regulator and the frequency,of the inverter is controlled by feedback means in response to the loadvoltage, the feedback means and inverter are adapted to drive thefrequency above the critical upon failure of the feedback means. isthereby limited to a safe value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of aferroresonant inverter circuit of the prior art, of the type to whichthis invention may be applied;

FIG. 2 is a plot of the ferroresonant circuit frequency characteristicthat is used in this invention;

FIG. 3 is a schematic diagram of a ferroresonant inverter circuit thatis particularly adapted to use this invention; and

FIG. 4 is a schematic diagram of a useful variation of the ferroresonantinverter circuit of FIG. 3.

DETAILED DESCRIPTION FIG. 1 is a simple block diagram of a regulatedinverter circuit as described in the Kakalec application previouslymentioned. An inverter 10 drives a ferroresonant regulator 11 to producea regulated output voltage at output terminal 12. To provide closed loopregulation, a voltage to frequency converter 13 senses the voltage atoutput terminal 12 and varies the frequency of inverter 10 in responsethereto. For purposes of this invention, regulator 11 may be connecteddirectly to the output to provide alternating current, or a rectifiermay be inserted to provide direct current. If the output is ac, thevoltage to frequency converter 13 may include its own rectifier.Furthermore, voltage to frequency converter 13 may include an oscillatorto drive inverter 10, or it may include impedances that become part ofthe frequency determining circuit of inverter 10, as exemplified by theKakalec application. To provide the negative feedback required forregulation, an increase in output voltage must of course produce adecrease in frequency of inverter 10.

The protection from overvoltage at output terminal 12 that is thesubject of this invention depends upon a characteristic of ferroresonantregulators that has jump frequency The output voltage previously beenunknown or not. understood. It has long been known that ferroresonantregulators have a limited frequency range for reliable operation andthat even within the range, the output voltage is frequency sensitive.Since the object of a regulator is a constant output voltage, theapplication of ferroresonant regulators has normally been limited tofixed frequency inputs such as 60 Hertz power lines. The emphasis hasbeen on keeping the input frequency constant, so that the particularfrequency characteristic has not been important. We have found, however,that the frequency characteristic of the ferroresonant regulator ispredictable, reliable and particularly useful.

This characteristic is illustrated by the curve of FIG. 2. The normaloperation of the ferroresonant regulator is described by a line 20 inFIG. 2. In fact, the normal operating range of a ferroresonant regulatoris shown as zone A. In this range, voltage and frequency are linearlyrelated; 10 percent increase in frequency is accompanied byapproximately 10 to 17 percent increase in output voltage. If thefrequency is elevated beyond this range however, it will eventuallyreach a point 21 at which the voltage suddenly drops a considerableamount to a point 22.

This discontinuity in the output voltage characteristics is caused by achange in the mode of operation.

In normal ferroresonant operation, of both the single core and thetwo-core types, an iron core in the output winding saturates every halfcycle of input voltage. When the core saturates, the impedance of theoutput winding drops to provide a low impedance path for the dischargeof the ferrocapacitor that is connected effectively across it. Since thevolt-time integral of output winding voltage required to saturate thecore is a constain, the ferrocapacitor charges to a constant regulatedvoltage, provided the driving frequency is constant, in spite of changesininput voltage. When the driving frequency rises so high however, thatthe core fails to saturate before the input voltage reverses, no lowimpedance path is provided to reverse the charge on the ferrocapacitor.The voltage generated in the output winding then opposes the capacitorvoltage for a portion of the cycle, and a greatly reduced output voltageresults. Furthermore, since the ferrocapacitor and the output windingare each effectively connected across the output, the reduced outputvoltage amounts to a reduced output winding voltage. The reduced outputwinding voltage would require even longer to saturate the core, andstable operation in a low output mode results.

Once having exceeded the critical frequency at point 21, therefore, theregulator operates in a stable nonsaturating mode along line 23. In thismode, an increase in frequency is accompanied by a decrease in outputvoltage. In order to return operation to the normal mode, the frequencymust be considerably reduced to a point 24, at which the core will againsaturate and the output voltage will once again jump up to follow line20. Operation between these points on curves 20 and 23 is quite stableand reliable.

According to the principles of this invention, the voltage regulatingfeedback loop of the system' is arranged so that a failure that wouldnormally tend to raise the output voltage beyond a safe level will causethe ferroresonant regulator to drop immediately into the nonsaturatingmode, hence reducing output voltage. This can be readily illustratedwith reference to the dc to dc converter of FIG. 3, which is shown anddescribed in the above-mentioned Kakalec application.

The dc to dc converter of FIG. 3 includes an inverter 10, aferroresonant regulated rectifier l 1, and a voltage to frequencyconverter 13. Inverter is of the wellknown push-pull Jensen type thatwould normally be free running. The drive for each of the transistors issupplied by separate transformer windings 27 and 28.

.The operation of this type inverter is well known and need not bedescribed here.

F erroresonant regulated rectifier 11 is also of a wellknown type. Anoutput winding 29, wound upon a saturating core portion 31, is shuntedby a ferrocapacitor 32. Magnetic shunts 33-33 provide decoupling of coreportion 31 from the inverter part 30 of the core to allow saturation ofthe former without saturating the latter. The ac terminals of afull-wave bridge rectifier 34 are connected across a portion of outputwinding 29 and the dc output of bridge 34 is connected to a pair ofoutput terminals 36 and 37, rspectively. A capacitor 38 may be connectedacross the output terminals to provide filtering. Core portion3l"saturates every half cycle at a fixed volt-time integral to reversethe voltage on ferrocapacitor 32 in typical ferroresonant regulatoroperation.

Voltage to frequency converter 13 includes a frequency determiningnetwork 41 and an error detector 42. Frequency determining network 41operates in a manner that simulates saturation of the core of theinverter. An RC integrating network including resistors 43 and 44 andcapacitor 45 is connected across secondary winding 46. Also connectedacross winding 46 is the series combination of an inductor 47 and atriac 48. Paired zener diodes 49 and 51 are connected in seriesopposition from the gate lead of triac 48 across integrating capacitor45. r

Frequency determining network 41 operates as follows: integratingcapacitor 45 charges at a rate that is a function of the volt-timeintegral of the voltage across secondary winding 46. When capacitor 45charges to a high enough voltage of either polarity, diode 49 or 51,whichever is at the time back-biased, breaks down to send a pulse ofcurrent into the gate of triac 48. The triac thereupon turns ON,switching inductor 47 directly across winding 46. Inductor 47 representsa relatively low impedance, effectively short circuiting winding 46 tosimulate saturation of core portion 30, thereby terminating the halfcycle of inverter 10 and starting a new half cycle.

The frequency of inverter 10 is therefore determined by the amount oftime required for capacitor 45 to reach the breakdown potential of zenerdiodes 49 and 51. This time is controlled by the zener potential ofdiodes 49 and 51, the capacitance of capacitor 45, the resistance ofresistors 43 and 44 and the voltage across winding 46.

Error detector 42 is applied to control the frequency of inverter 10 byvarying the effective resistance of resistor 43. Error detector 42includes a zener diode 52 connected in series with a biasing resistor 53across output terminals 36 and 37 to provide a constant referencepotential at the emitter of an error detector transistor 54. Apotentiometer 56 is also connected across the output terminals, and itstap is connected to the base of error detector transistor 54. The dcterminals of a full-wave bridge rectifier 57 are connected betweenemitter and collector of transistor 54, and the ac terminals areconnected across resistor 43.

If the output voltage tends to increase, the portion of output volt-agesensed by the tap on potentiometer 56 increases, causing the base oferror detector transistor 54 to become less positive with respect to itsemitter. Transistor 54 thereupon becomes less conductive and representsahigher impedance shunt across resistor 43. This in turn has the effectof increasing the time con- 'stant of the integrating circuit andtherefore increasing the time until triac 48 tires to end the half cycleof inverter 10. The resulting lower frequency drive to ferunshunted, andthe frequency of inverter drops to its minimum. The output voltagetherefore will not rise. If transistor 54fails shorted, however,resistor 43 is effectively shorted, and the frequency of inverter 10rises to its maximum. Contrary to usual design philosophy, which wouldminimize this increase of frequency, the principles of this inventiondictate that in the case of a feedback circuit fault that tends toincrease frequency, the increase be great enough to drive the frequencywell above the critical frequency of point 21 in FIG. 2. The outputvoltage is reduced because the ferroresonant regulator is driven intoits nonsaturating mode.

.The critical jump frequency of any given ferroresonant regulator can bedetennined by a straight-forward measurement. The frequency of thesignal driving the regulator is slowly raised until the sudden jump inoutput voltage is observed. Since a higher input voltage causes the coreto saturate more quickly, it also requires a higher frequency to reachthe jump point of nonsaturation. The critical jump frequency shouldtherefore be determined under the conditions of both minimum (low line)and maximum (high line) input voltage.

The feedback loop of the system is then adjusted so that under low lineconditions, which produce the lowest jump frequency, full load currentcan be drawn without exceeding the jump frequency. At the same time,under high line conditions and light load a shorted feedback circuitmust drive the frequency above the jump point, which is at its highestunder these conditions.

In the case of the circuit of FIG. 3, adjustment of the feedback loop toassure overvoltage protection may be accomplished by the choice ofvalues for resistors 43 and 44. Resistor 44 determines the frequency ofoperation when resistor 43 is shorted. The value of resistor 44 must below enough to drive the frequency above the jump frequency at high lineand light load. It must, 'of course, be large enough in value, however,to keep inverter 10 oscillating so that no damage occurs to theinverter. Resistor 43 is chosen so that with both resistors 43 and 44 inthe circuit, the frequency at full ldad and low line will not exceed thejump point. It has been found that ferroresonant regulators designedwith a resonant frequency of approximately percent above nominaloperating frequency will have a no-load jump frequency approximately twotimes the no-load operating frequency. At full load, the jump frequencywill be considerably less. A frequency increase of three or four timesupon feedback-loop failure, therefore, will insure the benefits of thisinvention. Since they determine the frequency of inverter 10, resistors43 and 44 also affect output voltage. The number of turns of winding 29may therefore have to be adjusted to achieve the desired output voltagewith a satisfactory value of resistors 43 and 44.

It is important to observe that although one can determine maximumoutput voltage by slowly raising the frequency or increasing the loadthrough the jump point, the self-protecting feature of this inventiondoes not require that the voltage of the jump point (21 of FIG. 2)actually be reached. When a fault occurs that terminates a single halfcycle of regulator input before the core can saturate, the outputvoltage immediately drops, and the regulator settles into thenonsaturating mode without approaching the voltage of point 21.

Positive protection from overvoltage at the output of a ferroresonantinverter circuit has therefore been obtained according to the principlesof this invention without the addition of parts, space or cost.

An alternative frequency determining network 41, which maybe substitutedfor that shown in FIG. 3, is shown in FIG. 4. This network utilizes theleakage inductance between windings 46 and 58 in place of the separateinductor 47. Triac 48 is connected directly across winding 46 and shortsthe winding when fired by the voltage across capacitor 45. According towellknown transformer theory, the short circuit is reflected through thetransformer as a short circuit in series with the leakage inductance. Inorder to simulate a saturated core to the Jensen inverter 10, theleakage inductance must be within the proper range. It has been foundthat if winding 46 is coupled too tightly to the other core windings 27,28, or 58, the inverter action is stifled. Magnetic shunts can, ofcourse, be used to isolate winding 46. Alternatively, optimum couplinghas been achieved when winding 46 is adjacent to the other windings onthe same core leg. This results in a short, sharp pulse of triaccurrent, good inverter action, and high efficiency.

This invention is of course not limited to the circuits of FIGS. 3 and4. Other systems that operate according to the block diagram of FIG. 1,so that a failure of the feedback loop can cause an unacceptable rise inoutput voltage, can be adapted to take advantage of these teachings. Theoperating point of the ferroresonant regulator circuit may be set closeto the critical jump frequency, or the feedback loop may be adapted todrive the frequency above the jump frequency when the anticipatedfailure occurs.

What is claimed is:

l. A self-limiting voltage regulating system for supplying a regulatedvoltage to a load comprising a source of ac signals of controllablefrequency, a ferroresonant regulator having a saturating core, saidferroresonant regulator connected to be driven by said ac signal and tosupply said load and feedback means including an amplifier connectedbetween said load and said ac signal source for controlling thefrequency of said ac signals in response to the voltage across saidload, said feedback means also including means for increasing thefrequency of said ac signals higher than will allow said core tosaturate when said amplifier is short-circuited.

2. A voltage regulating system as in claim 1 wherein said source of acsignals includes a transformer having a magnetic core and said feedbackmeans includes a winding on said core and an inductor, said inductorbeing periodically switched across said winding to control the frequencyof said signal.

3. A voltage regulating system as in claim 1 wherein said source of acsignals includes a transformer having a magnetic core and said feedbackmeans includes a winding on said core, said winding being periodicallyshorted to control the frequency of said signal.

4. A voltage regulating system as in claim 1 wherein theshort-circuiting of said amplifier at least doubles the frequency ofsaid ac signal.

' 5. A voltage regulating system for supplying a regulated voltage to aload comprising a ferroresonant regulator having a saturating core andan output winding on said core, said output winding being coupled tosaid load, a source of ac signals connected to said ferroresothe voltageacross said load, said feedback means including a timing capacitor, aresistance in series with said timing capacitor and a transistorconnected across at least a portion of said resistance, said portionbeing sufficiently large to increase the frequency of said signalshigher than will allow said core to saturate when said transistor isshorted, whereby the voltage across said load is limited.

1 i i i

1. A self-limiting voltage regulating system for supplying a regulatedvoltage to a load comprising a source of ac signals of controllablefrequency, a ferroresonant regulator having a saturating core, saidferroresonant regulator connected to be driven by said ac signal and tosupply said load and feedback means including an amplifier connectedbetween said load and said ac signal source for controlling thefrequency of said ac signals in response to the voltage across saidloAd, said feedback means also including means for increasing thefrequency of said ac signals higher than will allow said core tosaturate when said amplifier is short-circuited.
 2. A voltage regulatingsystem as in claim 1 wherein said source of ac signals includes atransformer having a magnetic core and said feedback means includes awinding on said core and an inductor, said inductor being periodicallyswitched across said winding to control the frequency of said signal. 3.A voltage regulating system as in claim 1 wherein said source of acsignals includes a transformer having a magnetic core and said feedbackmeans includes a winding on said core, said winding being periodicallyshorted to control the frequency of said signal.
 4. A voltage regulatingsystem as in claim 1 wherein the short-circuiting of said amplifier atleast doubles the frequency of said ac signal.
 5. A voltage regulatingsystem for supplying a regulated voltage to a load comprising aferroresonant regulator having a saturating core and an output windingon said core, said output winding being coupled to said load, a sourceof ac signals connected to said ferroresonant regulator to drive saidcore periodically into saturation, and voltage to frequency feedbackmeans connected between said load and said source of ac signals tocontrol the frequency of said signals in response to the voltage acrosssaid load, said feedback means including a timing capacitor, aresistance in series with said timing capacitor and a transistorconnected across at least a portion of said resistance, said portionbeing sufficiently large to increase the frequency of said signalshigher than will allow said core to saturate when said transistor isshorted, whereby the voltage across said load is limited.